Most people make GPX files from their GPS units, if you have a GPS or an iPhone you may be able to make GPX traces. If you don't have those technologies, you can edit maps directly from aerial imagery. The above link should be able to help you with both approaches.

A GPX file is simply the chosen file format that the OSM website uses alongside other map making tools such as JOSM. There are lots of manufacturers of GPS devices which record the waypoints and tracks in different ways.

To allow the data from all these different devices to provide a uniform data file format to the OSM website, there are free tools ( GPSBABEL to name one which I and many others use ) which will download the data from your GPS and
convert that into a standard GPX file.

Most GPS devices ( and I am only familiar with the Garmin units ) will log continuously where you are, for example I set my Garmins to log once a second. So after an hour of walking or driving you will have 3600 data points of the route you took which maybe a path or road.

This recorded track can then be downloaded by you to your personal computer and you can use tools such as JOSM to trace over the GPX track and upload a new road or path.

If you have been to a new area that has not been mapped it is worthwhile uploading the GPX trail to the OSM website putting the route of the road into the public domain.

If a number of people travel down the same route, and they all upload their GPX tracks, a good average of the route of road can be seen as a series of overlayed GPX traces, thus improving the accuracy of the drawn road on OSM.

Some mappers might only drive a road and upload their GPX trails, other mappers can then view your trail and trace over the trail ( though this is less usual as it cannot be know simply from the GPX trail what the route is ( path / road / major highway etc ).